arXiv subjects

Fabian Heitsch

Publications and source records attributed to Fabian Heitsch.

At least 37 records · Page 2Linked to original sources

arXiv2014

Accretion and Diffusion Timescales in Sheets and Filaments

A comparison of accretion and (turbulent) magnetic diffusion timescales for sheets and filaments demonstrates that dense star-forming clouds generally will -- under realistic conditions -- become supercritical due to mass accretion on timescales at least an order of magnitude shorter than ambipolar and/or turbulent diffusion timescales. Thus, ambipolar or turbulent diffusion -- while present -- is unlikely to control the formation of cores and stars.

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arXiv2013

Gravitational Infall onto Molecular Filaments II. Externally Pressurized Cylinders

In an extension of Fischera & Martin (2012a) and Heitsch (2013), two aspects of the evolution of externally pressurized, hydrostatic filaments are discussed. (a) The free-fall accretion of gas onto such a filament will lead to filament parameters (specifically, FWHM--column density relations) inconsistent with the observations of Arzoumanian et al. (2011), except for two cases: For low-mass, isothermal filaments, agreement is found as in the analysis by Fischera & Martin (2012b). Magnetized cases, for which the field scales weakly with the density as $B\propto n^{1/2}$, also reproduce observed parameters. (b) Realistically, the filaments will be embedded not only in gas of non-zero pressure, but also of non-zero density. Thus, the appearance of sheet-embedded filaments is explored. Generating a grid of filament models and comparing the resulting column density ratios and profile shapes with observations suggests that the three-dimensional filament profiles are intrinsically flatter than isothermal, beyond projection and evolution effects.

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arXiv2013

Gravitational Infall onto Molecular Filaments

Two aspects of filamentary molecular cloud evolution are addressed: (1) Exploring analytically the role of the environment for the evolution of filaments demonstrates that considering them in isolation (i.e. just addressing the fragmentation stability) will result in unphysical conclusions about the filament's properties. Accretion can also explain the observed decorrelation between FWHM and peak column density. (2) Free-fall accretion onto finite filaments can lead to the characteristic "fans" of infrared-dark clouds around star-forming regions. The fans may form due to tidal forces mostly arising at the ends of the filaments, consistent with numerical models and earlier analytical studies.

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arXiv2013

The Effects of Inhomogeneities within Colliding Flows on the Formation and Evolution of Molecular Clouds

Observational evidence from local star-forming regions mandates that star formation occurs shortly after, or even during, molecular cloud formation. Models of the formation of molecular clouds in large-scale converging flows have identified the physical mechanisms driving the necessary rapid fragmentation. They also point to global gravitational collapse driving supersonic turbulence in molecular clouds. Previous cloud formation models have focused on turbulence generation, gravitational collapse, magnetic fields, and feedback. Here, we explore the effect of structure in the flow on the resulting clouds and the ensuing gravitational collapse. We compare two extreme cases, one with a collision between two smooth streams, and one with streams containing small clumps. We find that structured converging flows lead to a delay of local gravitational collapse ("star formation"). Thus, more gas has time to accumulate, eventually leading to a strong global collapse, and thus to a high star formation rate. Uniform converging flows fragment hydrodynamically early on, leading to the rapid onset of local gravitational collapse and an overall low sink formation rate.

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arXiv2012

Aspect Ratio Dependence of the Free-Fall Time for Non-Spherical Symmetries

We investigate the collapse of non-spherical substructures, such as sheets and filaments, which are ubiquitous in molecular clouds. Such non-spherical substructures collapse homologously in their interiors but are influenced by an edge effect that causes their edges to be preferentially accelerated. We analytically compute the homologous collapse timescales of the interiors of uniform-density, self-gravitating filaments and find that the homologous collapse timescale scales linearly with the aspect ratio. The characteristic timescale for an edge driven collapse mode in a filament, however, is shown to have a square root dependence on the aspect ratio. For both filaments and circular sheets, we find that selective edge acceleration becomes more important with increasing aspect ratio. In general, we find that lower dimensional objects and objects with larger aspect ratios have longer collapse timescales. We show that estimates for star formation rates, based upon gas densities, can be overestimated by an order of magnitude if the geometry of a cloud is not taken into account.

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arXiv2012

Complex Structure in Class 0 Protostellar Envelopes III: Velocity Gradients in Non-Axisymmetric Envelopes, Infall or Rotation?

We present an interferometric kinematic study of morphologically complex protostellar envelopes based on observations of the dense gas tracers N2H+ and NH3. The strong asymmetric nature of most envelopes in our sample leads us to question the common interpretation of velocity gradients as rotation, given the possibility of projection effects in the observed velocities. Several "idealized" sources with well-ordered velocity fields and envelope structures are now analyzed in more detail. We compare the interferometric data to position-velocity diagrams of kinematic models for spherical rotating collapse and filamentary rotating collapse. For this purpose, we developed a filamentary parametrization of the rotating collapse model to explore the effects of geometric projection on the observed velocity structures. We find that most envelopes in our sample have PV structures that can be reproduced by an infalling filamentary envelope projected at different angles within the plane of the sky. The infalling filament produces velocity shifts across the envelope that can mimic rotation, especially when viewed at single-dish resolutions and the axisymmetric rotating collapse model does not uniquely describe any dataset. Furthermore, if the velocities are assumed to reflect rotation, then the inferred centrifugal radii are quite large in most cases, indicating significant fragmentation potential or more likely another component to the line-center velocity. We conclude that ordered velocity gradients cannot be interpreted as rotation alone when envelopes are non-axisymmetric and that projected infall velocities likely dominate the velocity field on scales larger than 1000 AU.

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arXiv2012

Very Large Array Observations of Ammonia in Infrared-Dark Clouds II: Internal Kinematics

Infrared-dark clouds (IRDCs) are believed to be the birthplaces of rich clusters and thus contain the earliest phases of high-mass star formation. We use the Green Bank Telescope (GBT) and Very Large Array (VLA) maps of ammonia (NH3) in six IRDCs to measure their column density and temperature structure (Paper 1), and here, we investigate the kinematic structure and energy content. We find that IRDCs overall display organized velocity fields, with only localized disruptions due to embedded star formation. The local effects seen in NH3 emission are not high velocity outflows but rather moderate (few km/s) increases in the line width that exhibit maxima near or coincident with the mid-infrared emission tracing protostars. These line width enhancements could be the result of infall or (hidden in NH3 emission) outflow. Not only is the kinetic energy content insufficient to support the IRDCs against collapse, but also the spatial energy distribution is inconsistent with a scenario of turbulent cloud support. We conclude that the velocity signatures of the IRDCs in our sample are due to active collapse and fragmentation, in some cases augmented by local feedback from stars.

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arXiv2011

Rapid Star Formation and Global Gravitational Collapse

Most young stars in nearby molecular clouds have estimated ages of 1-2 Myr, suggesting that star formation is rapid. However, small numbers of stars in these regions with inferred ages of >= 5-10 Myr have been cited to argue that star formation is instead a slow, quasi-static process. When considering these alternative pictures it is important to recognize that the age spread in a given star-forming cloud is necessarily an upper limit to the timescales of local collapse, as not all spatially-distinct regions will start contracting at precisely the same instant. Moreover, star-forming clouds may dynamically evolve on timescales of a few Myr; in particular, global gravitational contraction will tend to yield increasing star formation rates with time due to generally increasing local gas densities. We show that two different numerical simulations of dynamic, flow-driven molecular cloud formation and evolution 1) predict age spreads for the main stellar population roughly consistent with observations, and 2) raise the possibility of forming small numbers of stars early in cloud evolution, before global contraction concentrates the gas and the bulk of the stellar population is produced. In general, the existence of a small number of older stars among a generally much-younger population is consistent with the picture of dynamic star formation, and may even provide clues to the time evolution of star-forming clouds.

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arXiv2011

Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping

We present a study of dense molecular gas kinematics in seventeen nearby protostellar systems using single-dish and interferometric molecular line observations. The non-axisymmetric envelopes around a sample of Class 0/I protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to construct line-center velocity and linewidth maps for all sources to examine the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000 AU. The direction of the large-scale velocity gradients from single-dish mapping is within 45 degrees of normal to the outflow axis in more than half the sample. Furthermore, the velocity gradients are often quite substantial, the average being ~2.3 km\s\pc. The interferometric data often reveal small-scale velocity structure, departing from the more gradual large-scale velocity gradients. In some cases, this likely indicates accelerating infall and/or rotational spin-up in the inner envelope; the median velocity gradient from the interferometric data is ~10.7 km/s/pc. In two systems, we detect high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\ emission. This enables us to study the infall and rotation close to the disk and estimate the central object masses. The velocity fields observed on large and small-scales are more complex than would be expected from rotation alone, suggesting that complex envelope structure enables other dynamical processes (i.e. infall) to affect the velocity field.

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arXiv2011

Gravity or turbulence? II. Evolving column density PDFs in molecular clouds

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior occurs when the cloud is dominated by gravity. In the present contribution we use thermally bi-stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal Npdf, and later, in a power-law Npdf. We thus suggest that the observed Npdf of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power-law slope is not unique, as it depends on the projection in which the cloud is being observed.

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arXiv2011

Flow-Driven Cloud Formation and Fragmentation: Results From Eulerian and Lagrangian Simulations

The fragmentation of shocked flows in a thermally bistable medium provides a natural mechanism to form turbulent cold clouds as precursors to molecular clouds. Yet because of the large density and temperature differences and the range of dynamical scales involved, following this process with numerical simulations is challenging. We compare two-dimensional simulations of flow-driven cloud formation without self-gravity, using the Lagrangian Smoothed Particle Hydrodynamics (SPH) code VINE and the Eulerian grid code Proteus. Results are qualitatively similar for both methods, yet the variable spatial resolution of the SPH method leads to smaller fragments and thinner filaments, rendering the overall morphologies different. Thermal and hydro-dynamical instabilities lead to rapid cooling and fragmentation into cold clumps with temperatures below 300K. For clumps more massive than 1 Msun/pc, the clump mass function has an average slope of -0.8. The internal velocity dispersion of the clumps is nearly an order of magnitude smaller than their relative motion, rendering it subsonic with respect to the internal sound speed of the clumps, but supersonic as seen by an external observer. For the SPH simulations most of the cold gas resides at temperatures below 100K, while the grid-based models show an additional, substantial component between 100 and 300K. Independently of the numerical method our models confirm that converging flows of warm neutral gas fragment rapidly and form high-density, low-temperature clumps as possible seeds for star formation.

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arXiv2010

Formation of Cold Filamentary Structure from Wind Blown Superbubbles

The expansion and collision of two wind-blown superbubbles is investigated numerically. Our models go beyond previous simulations of molecular cloud formation from converging gas flows by exploring this process with realistic flow parameters, sizes and timescales. The superbubbles are blown by time-dependent winds and supernova explosions, calculated from population synthesis models. They expand into a uniform or turbulent diffuse medium. We find that dense, cold gas clumps and filaments form naturally in the compressed collision zone of the two superbubbles. Their shapes resemble the elongated, irregular structure of observed cold, molecular gas filaments and clumps. At the end of the simulations, between 65 and 80 percent of the total gas mass in our simulation box is contained in these structures. The clumps are found in a variety of physical states, ranging from pressure equilibrium with the surrounding medium to highly under-pressured clumps with large irregular internal motions and structures which are rotationally supported.

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arXiv2010

Gravity or Turbulence? The velocity dispersion-size relation

We discuss the nature of the velocity dispersion vs. size relation for molecular clouds. In particular, we add to previous observational results showing that the velocity dispersions in molecular clouds and cores are not purely functions of spatial scale but involve surface gas densities as well. We emphasize that hydrodynamic turbulence is required to produce the first condensations in the progenitor medium. However, as the cloud is forming, it also becomes bound, and gravitational accelerations dominate the motions. Energy conservation in this case implies $|E_g| \sim E_k$, in agreement with observational data, and providing an interpretation for two recent observational results: the scatter in the $δv-R$ plane, and the dependence of the velocity dispersion on the surface density ${δv^2/ R} \propto Σ$. We argue that the observational data are consistent with molecular clouds in a state of hierarchical gravitational collapse, i.e., developing local centers of collapse throughout the whole cloud while the cloud itself is collapsing, and making equilibrium unnecessary at all stages prior to the formation of actual stars. Finally, we discuss how this mechanism need not be in conflict with the observed star formation rate.

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arXiv2010

Morphological Complexity of Protostellar Envelopes

Extinction maps at 8 micron from the Spitzer Space Telescope show that many Class 0 protostars exhibit complex, irregular, and on-axisymmetric structure within the densest regions of their dusty envelopes. Many of the systems have highly irregular and on-axisymmetric morphologies on scales $\sim$1000 AU, with a quarter of the sample exhibiting filamentary or flattened dense structures. Complex envelope structure is observed in regions spatially distinct from outflow cavities, and the densest structures often show no systematic alignment perpendicular to the cavities. We suggest that the observed envelope complexity is the result of collapse from protostellar cores with initially non-equilibrium structures. The striking non-axisymmetry in many envelopes could provide favorable conditions for the formation of binary systems. We then show that the kinematics around L1165 as probed with N2H+ are indicative of asymmetric infall; the velocity gradient is not perpendicular to the outflow.

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arXiv2010

Competitive Accretion in Sheet Geometry and the Stellar IMF

We report a set of numerical experiments aimed at addressing the applicability of competitive accretion to explain the high-mass end of the stellar initial mass function in a sheet geometry with shallow gravitational potential, in contrast to most previous simulations which have assumed formation in a cluster gravitational potential. Our flat cloud geometry is motivated by models of molecular cloud formation due to large-scale flows in the interstellar medium. The experiments consisted of SPH simulations of gas accretion onto sink particles formed rapidly from Jeans-unstable dense clumps placed randomly in the finite sheet. These simplifications allow us to study accretion with a minimum of free parameters, and to develop better statistics on the resulting mass spectra. We considered both clumps of equal mass and gaussian distributions of masses, and either uniform or spatially-varying gas densities. In all cases, the sink mass function develops a power law tail at high masses, with $dN/dlog M \propto M^{-Γ}$. The accretion rates of individual sinks follow $\dot{M} \propto M^2$ at high masses; this results in a continual flattening of the slope of the mass function towards an asymptotic form $Γ\sim 1$ (where the Salpeter slope is $Γ= 1.35$). The asymptotic limit is most rapidly reached when starting from a relatively broad distribution of initial sink masses. In general the resulting upper mass slope is correlated with the maximum sink mass; higher sink masses are found in simulations with flatter upper mass slopes. Although these simulations are of a highly idealized situation, the results suggest that competitive accretion may be relevant in a wider variety of environments than previously considered, and in particular that the upper mass distribution may generally evolve towards a limiting value of $Γ\sim 1$.

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arXiv2010

Spitzer View of Young Massive Stars in the LMC HII Complexes. II. N159

The HII complex N159 in the Large Magellanic Cloud (LMC) is used to study massive star formation in different environments, as it contains three giant molecular clouds (GMCs) that have similar sizes and masses but exhibit different intensities of star formation. We identify candidate massive young stellar objects (YSOs) using infrared photometry, and model their SEDs to constrain mass and evolutionary state. Good fits are obtained for less evolved Type I, I/II, and II sources. Our analysis suggests that there are massive embedded YSOs in N159B, a maser source, and several ultracompact HII regions. Massive O-type YSOs are found in GMCs N159-E and N159-W, which are associated with ionized gas, i.e., where massive stars formed a few Myr ago. The third GMC, N159-S, has neither O-type YSOs nor evidence of previous massive star formation. This correlation between current and antecedent formation of massive stars suggests that energy feedback is relevant. We present evidence that N159-W is forming YSOs spontaneously, while collapse in N159-E may be triggered. Finally, we compare star formation rates determined from YSO counts with those from integrated H-alpha and 24 micron luminosities and expected from gas surface densities. Detailed dissection of extragalactic GMCs like the one presented here is key to revealing the physics underlying commonly used star formation scaling laws.

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arXiv2010

Modelling Shear Flows with SPH and Grid Based Methods

Given the importance of shear flows for astrophysical gas dynamics, we study the evolution of the Kelvin-Helmholtz instability (KHI) analytically and numerically. We derive the dispersion relation for the two-dimensional KHI including viscous dissipation. The resulting expression for the growth rate is then used to estimate the intrinsic viscosity of four numerical schemes depending on code-specific as well as on physical parameters. Our set of numerical schemes includes the Tree-SPH code VINE, an alternative SPH formulation developed by Price (2008), and the finite-volume grid codes FLASH and PLUTO. In the first part, we explicitly demonstrate the effect of dissipation-inhibiting mechanisms such as the Balsara viscosity on the evolution of the KHI. With VINE, increasing density contrasts lead to a continuously increasing suppression of the KHI (with complete suppression from a contrast of 6:1 or higher). The alternative SPH formulation including an artificial thermal conductivity reproduces the analytically expected growth rates up to a density contrast of 10:1. The second part addresses the shear flow evolution with FLASH and PLUTO. Both codes result in a consistent non-viscous evolution (in the equal as well as in the different density case) in agreement with the analytical prediction. The viscous evolution studied with FLASH shows minor deviations from the analytical prediction.

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arXiv2009

Spitzer and HHT observations of starless cores: masses and environments

We present Spitzer observations of a sample of 12 starless cores selected to have prominent 24 micron shadows. The Spitzer images show 8 and 24 micron shadows and in some cases 70 micron shadows; these spatially resolved absorption features trace the densest regions of the cores. We have carried out a 12CO (2-1) and 13CO (2-1) mapping survey of these cores with the Heinrich Hertz Telescope (HHT). We use the shadow features to derive optical depth maps. We derive molecular masses for the cores and the surrounding environment; we find that the 24 micron shadow masses are always greater than or equal to the molecular masses derived in the same region, a discrepancy likely caused by CO freeze--out onto dust grains. We combine this sample with two additional cores that we studied previously to bring the total sample to 14 cores. Using a simple Jeans mass criterion we find that ~ 2/3 of the cores selected to have prominent 24 micron shadows are collapsing or near collapse, a result that is supported by millimeter line observations. Of this subset at least half have indications of 70 micron shadows. All cores observed to produce absorption features at 70 micron are close to collapse. We conclude that 24 micron shadows, and even more so the 70 micron ones, are useful markers of cloud cores that are approaching collapse.

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